Reverse osmosis systems for supplying purified water for home use have been proposed wherein a purified water storage container is used with a barrier such as an impermeable flexible diaphragm separating the purified water from pressurized reject water. U.S. Pat. No. 4,176,063 issued to Tyler illustrates such an arrangement. A modified form of the same type system is found in related Tyler U.S. Pat. No. 4,391,712. U.S. Pat. No. 4,657,674 to Burrows also shows an arrangement wherein the purified water storage container is pressurized by reject water separated from the purified water by a membrane of extended area. In the systems shown in all of these patents a thin flexible membrane of large surface area constitutes the only separation between pressurized reject water and purified water. Such an arrangement poses the risk of contamination of the purified product by mass transfer of reject water through the material of the flexible membrane. There is also the danger of contamination through leakage or rupture through the single separating membrane. In addition, such prior systems have tended to rely on relatively complex multipart control valves. A typical control valve is illustrated in the above referenced Tyler patents.